1. Field of the Invention
The present invention relates to an apparatus and method for performing Hybrid Automatic Repeat reQuest (HARQ) transmission, and in particular, to an apparatus and method for optimizing a data rate by in a wireless communication system supporting HARQ.
2. Description of the Related Art
A Hybrid Automatic Repeat reQuest (HARQ) scheme is an advanced form of an Automatic Repeat reQuest (ARQ) scheme. In the ARQ scheme, erroneous and lost packets are retransmitted so that data reliability can be enhanced by reducing packet errors and loss. According to the ARQ scheme, an initially transmitted packet is retransmitted without alteration. In this case, information on previously transmitted packets is not utilized at all. Conversely, in the HARQ scheme, information on the previously transmitted packets is utilized for packet retransmission, thereby enhancing packet reliability.
The HARQ scheme may use either a Chase Combining (CC) scheme or an Incremental Redundancy (IR) scheme. In the CC scheme, a packet is detected by utilizing both information on a retransmitted packet and information on a previously transmitted packet. Therefore, when packet transmission is not successful, the same packet as an original packet is retransmitted to a receiving end. The original packet and the retransmitted packet are combined in the receiving end so as to detect the original packet. In this case, the receiving end can achieve a unique HARQ combining effect.
On the other hand, in the IR scheme, the original packet is transmitted at the time of initial transmission and then transmits redundancy information generated when the original packet is coded using the retransmitted packet. The redundancy information may be generated using various coding schemes such as a turbo coding scheme, a Rate Compatible Punctured Convolutional (RCPC) coding scheme, and a Low Density Parity Check (LDPC) coding scheme. For example, if the transmitting end employs the IR scheme, a redundancy information block is generated by coding the data to be transmitted and then the original packet is initially transmitted to the receiving end. Thereafter, if the original packet is not successfully transmitted, the transmitting end transmits to the receiving end the redundancy information block having the same length as the original packet. The transmitting end transmits the redundancy information block generated using any one of the aforementioned coding schemes, instead of transmitting the original packet, when the receiving end requests retransmission.
The receiving end may combine the original packet received from the transmitting end and the redundancy information block received during a retransmission process, to detect the original packet. In this case, the receiving end may achieve a unique HARQ combining effect as well as a coding gain.
As described above, if a wireless communication system uses the IR scheme, the transmitting end retransmits the redundancy information having the same length as the original packet when the receiving end requests retransmission. A packet transmission success rate varies depending on a wireless channel environment (e.g., Signal to Noise Ratio (SNR)) of the wireless network system. Therefore, the transmission success rate of respective retransmission packets varies depending on the wireless channel environment according to the conventional IR scheme in which redundancy information having a specific length is retransmitted. Accordingly, an optimal data rate cannot be achieved in the wireless communication system.